This invention relates to switches which control the speed of an electric motor in response to the position of the switch operator. More particularly this invention relates to trigger switches which are usually mounted in the handle of a portable electric appliance or tool and are operated by the index finger of the hand holding the appliance or tool to vary the speed of the electric motor thereof. Such switches generally comprise a depressible trigger member which upon initial depression operates to close switch contacts within the switch housing to connect the device to an electrical source, and upon further or subsequent depression operates a variable resistor to change the amount of resistance present in a speed control circuit contained within the housing. The speed control circuit comprises a thyristor and a heat sink therefore as well as a capacitor connected with the variable resistor to provide an RC timing circuit for controlling the thyristor. A switch of the foregoing type is a disclosed and claimed in U.S. Pat. No. 3,775,576 issued Nov. 27, 1973, to H. W. Brown, and assigned to the assignee of this invention, the disclosure of which is incorporated herein by reference.
A major consideration in providing switches of the aforementioned type is the ability to readily and economically assemble the components which comprise the speed control circuit. The comparatively small space available within the switch housing for packaging the components which comprise the speed control circuit and the amount of heat generated by the thyristor of that circuit requires the heat sinking capabilities to be highly efficient. The durability and reliability of the speed control circuit for such switches is significantly improved when the number of hand soldered wired connections between the components is minimized, thereby to also reflect an economic advantage in fabricating the speed control circuit. U.S. Pat. Nos. 4,137,490 issued Jan. 30, 1979 and 4,205,434 issued June 3, 1980, both to D. J. Brozoski et al and both assigned to the assignee of this invention disclose trigger speed control switches wherein the anode of a thyristor chip is soldered directly to the heat sink to provide highly efficient thermal conduction as well as an economically advantageous and reliable mechanical connection for the thyristor. These patents also disclose an improved method of attaching a capacitor chip to the speed control circuit and to the gate and cathode of the thyristor for reliable operation and economic assembly advantages. However, in each of the aforementioned patents the variable resistor comprises a separate resistor strip which is positioned in the housing along side, but spaced from, a collector which is an extension of the heat sink. A brush contactor carried by the trigger bridges the resistor and collector strip to vary the resistance in the speed control circuit according to the depth of trigger depression. Imperfections occurring in the manufacture of the resistor strip or in the formation of the collector or in mounting either of these elements within the switch housing can lead to undue and uneven wear of the brush contactor, resistor or collector elements. Manufacturing processes which attempt to hold closer tolerances for these elements increase the cost thereof. Accordingly while these prior switches have been useful for their intended purposes, the subject invention provides improvements thereover.